[Pharo-dev] Pharo and special unary selectors

Sven Van Caekenberghe sven at stfx.eu
Fri Nov 17 13:08:17 EST 2017


Parsing difficulty also means that it is harder for humans to understand, to explain to (new) users.

It would be pretty strange to have binary selectors that are unary, is my first reaction.

> On 17 Nov 2017, at 18:32, Thierry Goubier <thierry.goubier at gmail.com> wrote:
> 
> Le 17/11/2017 à 10:14, Nicolas Cellier a écrit :
>> 2017-11-17 17:40 GMT+01:00 Gabriel Cotelli <g.cotelli at gmail.com <mailto:g.cotelli at gmail.com>>:
>>    I would really like to see % removed as a binary selector and
>>    available to use in unary or keyword ones. The only implementor in a
>>    Pharo 6 image is:
>>      % aNumber
>>         ^ self \\ aNumber
>> +1, such alias has nothing to do in Kernel
>>    So it's juts aliasing \\ , and % most widespread usage in the real
>>    world is por percentages (the use in modular arithmetic is more a
>>    programming thing that math notation I think).
>>    And for allowing more Unicode code points for selector names I'm
>>    totally in for Symbols, Arrows, Math Symbols, etc... We just need to
>>    analyse the ones that makes sense as binary selectors, and the ones
>>    allowed for unary and keyword ones. This will allow us to write
>>    pretty cool DSLs.
>>    Just my opinion.
>> This could also be the case for punctuation like ! and ?
>> The idea of Torsten is more generic, any combination of binary char could be used.
>> From what I understand from https://en.wikipedia.org/wiki/LR_parser we would just have to scan one more token ahead for deciding if unary or binary, and could preserve our simple shift reduce steps...
> 
> The Smalltalk parsers being handwritten, there wouldn't be shift/reduces to contend with, and, anyway, the lexer doesn't shift/reduce; it would simply creates a token up to the next separator (that is goble up the next space/cr/end of line dot/closing parenthesis/etc...)
> 
>> So it seems doable for resolving the send.
> 
> Sort of. The parser difficulty would be this one:
> 
> anObject % print
> 
> Is this a binary selector with a print argument or two unary selectors?
> 
> Using the symbol table when you parse would solve it, but that is certainly not context free...
> 
>> More problematic would be the declaration of method, if we have both a unary + and a binary +, we will need new syntax for marking the difference.
> 
> In most cases, distinguishing between unary + declaration and binary + declaration would be doable:
> 
> + whatever
> 
> is the start of a binary selector
> 
> + ^ self
> 
> is the start (or the declaration of) an unary selector.
> 
> But writing
> 
> + self
> 
> Can be interpreted as either unary plus doing self, or binary + method definition...
> 
>> Whether it's worth or not is another matter...
> 
> Well, one should probably try to implement the various parsers for that (extend RB, extend the SmaCC Smalltalk parser, extend the Petit Parser) to see how much complexity it would bring.
> 
> Coming up with strange interpretations one could do with that syntax can be helpfull as well.
> 
> Regards,
> 
> Thierry
> 
>>    On Fri, Nov 17, 2017 at 6:32 AM, Torsten Bergmann <astares at gmx.de
>>    <mailto:astares at gmx.de>> wrote:
>>        Hi,
>>        just something to think about: one thing I always liked about
>>        Smalltalk is that it allows for nice DSL's. We have nice things
>>        like a unit framework in Pharo, ...
>>        In the most simple case one can easily implement own units just
>>        by providing a unary messages:
>>          1 m
>>          1 second
>>          1 px
>>          1 EUR
>>        One can easily implement an own Money class with a currency and
>>        then do polymorphic tricks like
>>           10 EUR + 20 EUR
>>        But we can currently can not implement special unary selectors
>>        (including special unary selectors with unicode) like:
>>           100 %
>>           20 $
>>           40 €
>>           12 ‰  (for promille)
>>        Especially things like 20 % would be nice for layout issues or
>>        other (Bloc comes to mind).
>>        Maybe we should put that on the roadmap of Pharo because IMHO it
>>        would be cool to support such things in the
>>        future. Dont know how much effort it currently means on the
>>        technical level but maybe others can comment.
>>        Thx
>>        T.
> 
> 





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